1. Field of the Invention
The present invention relates to a safety hypodermic syringe and, more particularly, to a needle holder positioning structure for safety hypodermic syringe.
2. Description of Related Art
In a safety hypodermic syringe, the plunger has an arrowhead-like front tip adapted for hooking the needle holder, for enabling the needle holder and the needle cannula at the needle holder to be pulled backwards to the inside of the barrel to prevent contamination after the service of the safety hypodermic syringe. The needle holder is press-fitted into the inside of the front section of the barrel. However, when installing the needle cannula in the needle holder, the backward pressure from the needle cannula tends to force the needle holder out of position. Further, in order to prohibit the needle holder from escaping out of the barrel from the front side of the barrel, the barrel is made having a conical front end. This conical front end design prohibits the needle holder from forward movement relative to the barrel, however it facilitates backward movement of the needle holder in the barrel during installation of the needle cannula in the needle holder. In order to ensure positive positioning of the needle holder in the barrel, the friction-fit between the needle holder and the inside wall of the barrel is enhanced. However, enhancing the friction-fit between the needle holder and the inside wall of the barrel complicates the installation of the needle holder in the barrel. Further, an injection molding mold for molding needle holders or barrels for safety hypodermic syringe has multiple cavities. Because the injection environment (fluid material filling position, injection temperature, cooling speed, etc.) in each cavity is different, it is difficult to control the dimensions of the injection-molded finished products precisely. A small specification tolerance may cause the needle holder and the barrel unable to match each other perfectly.
Therefore, it is desirable to provide a needle holder positioning structure for safety hypodermic syringe that eliminates the aforesaid problems.
The main object of the present invention is to provide a needle holder positioning structure for safety hypodermic syringe, which prevents falling of the needle holder to the inside of the barrel accidentally. It is another object of the present invention to provide a needle holder positioning structure for safety hypodermic syringe, which is easy and inexpensive to manufacture.
To achieve these and other objects of the present invention, the needle holder positioning structure for safety hypodermic syringe comprises a barrel, a needle holder, and a locating ring. The barrel comprises a fluid chamber. The fluid chamber has a front small inner diameter section, a rear big inner diameter section, a shoulder connected between the front small inner diameter section and the rear big inner diameter section, and an inside flange means disposed in the rear big inner diameter section adjacent to the shoulder. The needle holder comprises a tubular center base and a sleeve coaxially formed integral with the periphery of the tubular center base. The tubular center base has a retaining portion near a rear side thereof. The sleeve has a front engagement section press-fitted into the inside of the front small inner diameter section of the fluid chamber of the barrel and a springy rear expansion section. The rear expansion section has a front side edge corresponding to a back side of the shoulder of the barrel, and a rear side edge stopped at a front side of the inside flange means of the barrel. The rear expansion section defines with the periphery of the tubular center base a backwardly extended annular coupling chamber. The locating ring has an outside wall disposed in contact with an inside wall of the rear big inner diameter section of the fluid chamber of the barrel for enabling the locating ring to be moved axially in the barrel, and an inside wall disposed in contact with the periphery of the tubular center base of the needle holder for enabling the locating ring to be moved axially relative to the needle holder. The outside wall of the locating ring has a front section forming a neck adapted for engaging into the rear expansion section of the sleeve of the needle holder. The neck has an outer diameter not less than the inner diameter of the expansion section of the sleeve of the needle holder so as to expand the rear expansion section of the sleeve for enabling the rear side edge of the rear expansion section stopped at the front side of the inside flange means of the barrel. The locating ring further comprises a first retaining portion integral with a front part of the inside wall and adapted for engaging the retaining portion of the tubular center base of the needle holder, and a second retaining portion formed integral with a rear part of the inside wall and adapted for engaging the arrowhead-like front retainer tip of a plunger to be inserted into the fluid chamber of the barrel. When the neck of the locating ring inserted into the backwardly extended annular coupling chamber of the needle holder, the rear expansion section of the sleeve is expanded and positioned in the barrel between the shoulder and the inside flange means of the barrel, and therefore the needle holder is prohibited from backward movement relative to the barrel. Because this positioning design requires less precision, the manufacturing cost of the safety hypodermic syringe is low. Further, the flange, expansion section, retaining portions can be respectively formed of a unitary flange, or multiple protruded blocks, strips, protruded means of wedge-like, triangular, or arched cross section.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.